Well drilling equipment



Jan 13 1959 J. G. AscoTT ErAL 2,868,507 WELL DRLLING EQUIPMENT Fiied July 2e,

Jan. 13,l `1959 J. G. SCOTT Em 2,868,507 f WELL DRILLING EQUIPMENT SSheets-Shevet '2 Filed July A26, l-1954 Jam 13 1959 J. G. scoT'r x-:TAL 2,868,507

WELL DRILLING EQUIPMENT Filed July 26, 1954 3 Sheets-Sheet 3 FUELS;

FIIS Ei INVENTURS JAM/ ff G. 56077 @0L .4A/0 A Feet/MN BY R555@ 71 Hoz/:70N 27 29 wt; 'f/

James G. Scott, Simi, Roland A. Freeman, Santa Monica,

and Reese T. Houston, Simi, Calif., assignors to Amerii can Percussion Tool Company, San Francisco, Calif., a corporation of California Application July 26, 1954, Serial No. 445,666

Claims. (Cl. Z55- 4.6)

This invention relates generally to the drilling of oil, gas, water or like earth wells, and to equipment for such drilling operations.

In the past drills of the rotary type, used with mud circulation, have become standard for most earth drilling operations. Various types of drilling heads and cutters have been developed for diiferent formations and drilling conditions. Irrespective of the selection of drilling head, there has been a limitation with respect to the drilling speed for given conditions. For soft formations the cutting speed in a typical instance may be of the order of two to three feet per minute, but for hard rock formations the speed with the best cutting heads available may be of the order of one inch or less per hour. It will be apparent that drilling speed is one of the most important factors in the over-all cost of drilling a well, and that a method and equipment which permits a substantial increase in drilling speed, particularly for the harder formations, is of great economic importance. In addition to the matter of drilling speed conventional drilling techniques result in many lost or abandoned wells, and are severe upon the equipment. It is common for wells to be abandoned before completion due to in ability to cope with faulted or like unusual formations, or the difliculty or expense of operations for the removal of broken tools. Furthermore the drilling heads employed require frequent replacement and servicing, and this involves the lmany operations required to raise the head to the surface t Wear upon the cutters.

Another object of the invention is to provide a method and equipment of the above character which is characterized by greatly reduced wear upon the cutters for a given amount of eiective drilling, andfwhich in general greatly reduces Servicing and replacement requirements.

Another object of the invention is to provide a method and equipment of the above character which in general will result in a much higher percentage of completed wells, due particularly to the ability ofl the method and equipment to cope with unusual formations, and the greatly reduced breakage involved.

Another object of the invention is to provide a novel construction of earth drilling equipment which can be used with standard well drilling rigs, including rotary tables and mud circulating equipment.

Additional objects and features of the invention will appear from the following description in which the preferred embodiment of the invention has been set forth in detail in conjunction with the accompanying drawing.

Referring to the drawing:

Figure l is a side elevational View in section, illustrating a well equipment for carrying out the present method, and incorporating the equipment illustrated in Figures 2A-2C inclusive.

Figures 2A, 2B and 2C taken together, constitute a side elevational view of equipment incorporating the present invention, including the electrical actuator and associated parts.

Figure 3 is a cross sectional detail taken along the line 3-3 of Figure` 2B.

Figure 4 is a cross sectional detail taken along the line #Jl-4l of Figure 2C.

Figure 5 is a detail in section on an enlarged scale showing the sealing means adjacent the drilling head.

Figure 6 is a detail in section illustrating a portion of drill stem having tubular conductors for completing the electrical circuit.

Figure 7 is a circuit diagram for the electrical actuator.

In general the present invention involves the use of an electrical actuator comprising a magnetic eld which is excited with current pulses, and a core which is free to move 'in opposite directions and which is capable of delivering repeated impacts to an associated drilling head. rFhe construction is such that the actuator operates at resonant frequency, and its operation is maintined at a xed frequency irrespective of other factors involved in the drilling operation. In conjunction with receiving repeated impacts at a constant frequency, the cutter head and the assembly carrying the same is continuously rotated with circulation of mud for removal of cuttings.

AnLk assembly which may located within the well, and which includes an electrical actuator and the cutting head, as illustrated in Figures ZA-ZC inclusive.` it includes the drilling head lil carried at the lower end of the tubular housing 1l. Also the housing serves to mount the electrical actuator designated generally at l2. Its upper end is attached to the coupling member i3, which in turn forms the lower end of a rotary drilling stem.

The drilling head lt) in this instance consists generally of a body 16 carrying a plurality of toothed rotary cutter assemblies i7. Such drilling heads are well known to those familiar with oil and gas drilling methods, and are manufactured .in different sizes and types, for various kinds of drilling conditions. The threaded shank 1S of the head is engaged with the mounting member i9, which in turn is located within the lower part of the housing ll. The lower annular portion 2@ of member 19 is slidably received in the bushing 2l, which can be made of suitable material, such as bronze. A suitable torque transmitting connection is formed between these parts, and may consist of a plurality of splines 22 formed on the member 19, and slidably engaging slots 23 formed in the bushing 2l. The bushing 2l is attached by suitable means such as the screws 24, to the ring 26, and the latter being suitably secured, as by welding, to the lower end of the housing ll.

Av suitable seal is formed between the head lil and the ring 26 to prevent entrance of mud or cuttings. For this purpose we have shown a diaphragm 27 of ilexible material such as fabric reinforced natural or synthetic rubber, having its inner peripheral edge clamped to a gasket ring 28, and its outer margin clamped between the end piece of the ring 26 and the annular washer 29. The clamping washer 29 is held in place by the ring 3l., which is internally threaded and engaged with the threaded exterior of ring 26. The inner margin of the gasket ring 28 is tightly clamped between the shoulder 32 on the head and the opposed end face 33 of member 19.

Patentes aan. is, s V

The upper end of the member 19 is attached to the impact transmitting bar 34- which extends concentric with the axis of the housing 11. As will be.presently explained it is the upper end of this bar that receives impacts from the electrical actuator.

interposed between the member 19 and the electrical actuator there is a structure including guide means for the bar 34, and a recoil spring 62. As shown particularly in Figures 2B and 2C this structure consists of concentric tubes/36 and 37, which have their upper ends attached to a mounting ring 32, and their lower ends attached to a similar member 39. The mounting rings 38 and 39 are iixed to the housing 11 as by means of the weld connection d0. The space 41 between these tubes serves to conduct drilling mud and is connected with the mud receiving space 42. in the shank 18 of the drill head. For this purpose, space 41 is shown communi eating with a plurality of pipe ttinsg 43, which in turn are connected by the exible hose sections 44 with pipe fittings 46. The latter ittings directly connect through the member 19 to deliver mud into the space 42. In accordance with customary practice the cutter head is provided with ducts (not shown) which conduct the mud to the area being drilled, whereby chips or cuttings are removed.

The bar 34 is slidably iitted within the bushing 47, which in turn is iitted within the end casting 48. This casting has a lower flange 49 which is clamped to the flange 51 on the lower end of tube 37, by suitable means such as the screws S3. Suitable lubricant sealing means such `as the resilient O-ring seals 54 and S6, can be provided at the upper and lower ends of the bushing 47. Also suitable ttings 57 can be provided for the introduction of lubricant.

The lower end casting 48 seats an upper annular body 58, and the latter is bored and fitted with the bushing S9. The upper end of body 58 is provided with a flange 61 that overlies the ring 38.

The members 48 and S9 provide space for accommodating the loaded recoil spring 6?.. One end of this spring seats upon member 48, and the other upper end seats upon the collar 63, which is loosely itted about the bar 34. Collar 63 normally engages the annular shoulder 64 on the upper end of bushing 59 to resist the normal loading of the spring. immediately `above the collar 63 the rod 34 has an enlarged annular portion 66 to form the abutment shoulders 67 and 68. When no loading is being applied to the drill head, shoulder 67 is urged against the opposed face of the collar 63, by the force of the loaded spring 62.

The upper part 69 of the rod 34 forms what can be referred to as an anvil portion. It is slidably tted within a bushing 71 that is carried by the sleeve 72. The sleeve 72 is assembled within a bore 73 formed in the upper part of member 58, and is provided with a liange 74 `adapted to seat upon the cooperating shoulder face 76. The flange 74 is urged against the shoulder face 76 by suitable means such as a Belleville washer 77. A suitable arrangement of ducts 78 and 79 can be provided for introducing lubricant to the area between the bushing 71 and the anvil portion 69.

The electrical actuator is constructed as follows: It employs a ield comprising two electromagnet assemblies 12a and 12b, which serve to operate a central piston-like core 81. The `assemblies 12a and 12b can each consist of laminated cores S2 disposed radially about the axis ofthe central core, and provided with windings 83. The pole pieces S4 and 86 of each laminated core terminate at the boundary of a cylindrical passage 87, within which the central core 81 operates. rl`he two sets of laminated cores are shown separated by the mounting plate 88, and have their outer portions set interposed between the channels 89;, The passages 91 formed by these channels permit passage of mud. This arrangement bringsthe drilling mud into heat exchange relation with the magnetic cores and windings, thus providing for dissipation of heat. The lower ends of channels 89 are sealed with respect to ring 38, and passages 91 have their lower ends in free communication with the passage 41 between the tubes 36 and 37. Filler rings 92 are interposed between the pole pieces 84 and S6 of each electromagnet assembly, and the liller ring 93 is interposed between the two sets of pole pieces 86.

The central piston-like core 81 is provided with lower and upper extensions 94 and 95 of reduced diameter which form impact or hammer elements. The hammer element 94 delivers impacts'to the anvil portion 69 of the bar 34. A bearing assembly for element 95 can consist of a sleeve 96 which is mounted upon the lami` nated cores for the upper electromagnet assembly, and which serves to mount the bushing 97. This bushing is bored to receive the element 95, and is also provided with suitable means. such as ducts 98, for introducing lubricant. A similar bearing assembly is formed for the lower element 94. It consists of `a sleeve 99 (corresponding to sleeve 96) serving to mount the bushing 101 (corresponding to bushing 97). Here again the bushing can be provided with lubricant receiving ducts 102. At its lower end face the bushing 101 is in direct engagement with the upper end face of the sleeve 72. Also the lubricant ducts 102 can be in communication with ducts 78.

Overlying the upper electromagnet assembly 12a there is an annular plate 106 that is litted within the channels 39. Above this plate there is a second recoil spring 107' and mounting means -for the same. The lower end of spring 107 seats upon the impact receiving member 108, which has its end face 109y aligned with and opposed to the end face of the impact element 95. The member 108 is guided by the bushing ring 111 which is tted in the upper portion of the sleeve 96. The upper end of the spring 107 seats upon a member 112, which is maintained. in spaced relationship with the plate 106 by the tubular shroud 113 and the interposed plate 1-14; The inner) margin of the plate 114 forms an abutment shoulder 1115 for the thrust receiving member 108.

As will be presently explained the windings of the electromagnet assemblies are'energi'zed by current pulsations whereby the central core 81 -is caused to reciprocate a predetermined number of complete strokes per minute. The weight of the moving parts, the strength and preloading of the recoil springs, and the frequency of excitation of the windings is such that the core reciprocates at a constant frequency. When no downward force is being applied to the drill head, the force of the downward stroke of the core is absorbed by the lower recoil spring 62. However as the load is applied to the head an increasing `amount of the impactV energy is transmitted through the bar- 34 to the head and to the, formation, until a maximum optimum loading is reached, which is equal to the force of` the loaded spring 62; By way of example this may be of the order ofi 30,000 pounds.

A suitable circuit arrangementl for exciting the windings of theelectromagnet assemblies is shown in Figure 7. The lines L1 and L2 are connected to a suitable source of alternating current, such as volts at 30 cycles. The windings S3 have a common point of connection which connects with one line L1. The other terminals :of the windings 33 connect with the rectiers 1.16, which have `a common point of connection to the other line L2. With this arrangement rectifying pulses are supplied alternately to the two sets of windings, whereby for a currentrfrequency of 30 cycles per second, the centraly core 81 is` reciprocated at a frequency of 3600 complete strokes per minute, thus delivering 1800 Y impacts per minute.y The'rectiers 116 may be ofthe dry selenium type, the mercury vapor type, or any other type, suitable for the current carrying. capacity. required. `AReferring again toFigures 2A: and 2B, rectifier units 116 of the dry selenium type are shown disposed within -a space 117 provided for this purpose. This space is formed within a tubular housing 118 that is disposed in the upper portion of the main tubular housing 11. The lower lclosure 119 for the housing 118 can be provided with a sealed fitting 121 to accommodate the conducting ywires leading to the windings of the electromagnet assemblies. Also the end member 119 has a rigid sealed con- `nection with the member 112. Thus member 112 has an upwardly extending cylindrical portion 122, which has a threaded engagement 123 with la depending cylindrical portion 124 of the end closure 119. Suitable means suc-h as the O-ring seals 126 provide a fluid tight seal between these parts. The upper endsvof channels 89 are attached to the lower edge of a tubular section 127 that surrounds the Itubular portion 122, yand which abu'ts the lower end To carry out the method of the present invention, the

4 drill stem is rotated in accordance with conventional pracyface of the tubular portion 124. Also 'a ported seating ring 123 can be provided about the tubular section 127, and against which this section may seat.

To maintain the parts of the electrical actuator in assembled relation, we have shown a plurality of tie bolts 129, which `at one end engage the member 113, vand at the other end engage the member 58.

By reference to Figure 2B it will be seen that the annular passage 91 for circulating mud continues upwardly past the closed housing 118, to the space 131 which overlies the upper end 132 of the housing 118. Space 131 co-mmunicates with the lower end of the passage through the drill stem.

In Figure 1 we have shown how the equipment described above can be incorporated with other parts of a standard rotary drilling rig. The complete rig in this instance includes the rotary table 1 with its Kelly and motive means, the drill stem 2 which extends down into the well, a suitable derrick (not shown) provided with the usual crown and traveling blocks, and the drill stern swivel head 3 which is supported by the hoist hook 4. Also a suitable mud circulating system is provided, including a receiving sump and circulating pump, whereby mud is supplied to the head ofthe drill stem under pressure, delivered downwardly through the drill stem to the drill head, and then caused to circulate upwardly through the well casing to the surface of the well.

The apparatus described above (i. e. in Figures 2A-2C) has been designated by numeral 11a, and is interposed between the drill head and the lower end of the drill stem. A suitable two conductor cable is 4eX- tended down to the apparatus 11a, as fo-r example by threading it through the drill stem, `and at the top of the well this cable can connect with a slip ring connecting device 5, to permit making connections to an exterior cable 6. Cable 6 connects through the contro-l panel 7 with the alternating current generator 8, which'in turn can be driven by suitable motor means, such as a gasoline motor 9. The control panel can be provided with various control switches and instruments, including current, voltage 'and frequency meters.

In place of a cable extending down into the well we prefer to use the special drill stern construction shown in Figure 6. Thus the sections 136 are provided with inner tubular conductors 137 and 138 made of suitable material such as copper or aluminum. These conductors are insulated from the drill stem and from each other by the insulation 139 and 141. The inner wall of tube 13S can be provided with an abrasion resistant layer of insulation 142. such as a suitable synthetic rubber. The conductor tubes and the associated insulation terminate within the male and female parts 140 and. 143 of the stem couplers where they form flat end faces adapted to be compressed together to make good electrical connections.

Where permitted by conditions and applicable regulation, one side of the circuit may be conducted through the stem, thus permitting` use of a single insulated conducting tube.

tice, as for example at rotative speeds which may vary from 20 to 350 R. P. M., and at the same time the electrical actuator is energized by current from the generator 8, to apply impacts to the drill head ata relatively high and constant frequency, as for example the frequency of 1800 impacts per minute previously mentioned. The loading upon the drill head, that is, the force with which this head is permitted to bear upon the formation, is ad- ,iusted so that it is comparable to the loading required for maximum energy transfer. In the previously mentioned 'example this was assumed to be about 30,000 pounds.

When operated in the manner described above a number of remarkable characteristics will be noted. Assuming for example that the drill head is operating upon a relatively hard rock formation, the rate of drilling with the actuator in operation is many times the rate for the same rig operating under the same conditions but without operation of the actuator. By way of example where the drilling rate in a rock formation is of the order of l inch per hour, with the actuator in operation the drilling rate is of the orderof 10 feet or more per hour. This remarkable increase in drilling speed is obtained without excessive power consumption. Actually it appears that the torque required for turning the drill stem is somewhat less when the actuator is in operation, which is attributed to the freeing effect of the vibrations upon all of the rotating parts in the well, including the drill stem. Also this is attributed lto the fact'that instead of the drilling being due primarily to the torque of the head against the impact pattern.

formation, in accordance with our method it is due to the rapid and continued impacts upon the formation, with turning of the head serving primarily to distribute the The power consumption for the electrical actuator is relatively low, considering the remarkable increase in drilling speed accomplished.

Contrary to what might be expected wear upon the cutters is not proportional to the increase in drilling speed attained. In fact it appears that for a given amount of effective drilling, the wear upon the cutting teeth using our invention is substantially less than forl conventional rotary drilling.

In conventional rotary drilling practice it is common to encounter faulted and like difficult formations, which may make it necessary to abandon the well. It has been found that our 4drilling method and equipment is highly effective in drilling through such diiicult formations. Although faulting presents serious difficulties for conventional .drilling rigs, with our invention faulting has little if any effect upon the drilling efficiency, and the drilling head proceeds along .the desired direction without tending to cause breakage of the cutters or drilling head, and with little if any tendency toward deviation.

Conventional rotary drilling equipment is frequently susceptibleto an uneven or contoured drilling pattern upon the bottom formation, which greatly decreases drilling drilling eiiiciency, and which causes excessive wear and possible drill breakage. Here again our equipment is remarkably effective under such conditions. Where a formation is encountered which under normal co-nditions would develop such an uneven or contoured pattern, with our method and apparatus such a pattern does not develop, and at all times the pattern remains optimum for efficient drilling and for minimum possibility of drill breakage. This result is obtained without the use of expensive cutter heads such as have been developed for use in conventional rotary drilling, to minimize development of such patterns.

Under certain circumstances it may be desirable to correct an uneven or contoured drilling pattern, as for example where the pattern has been developed by use of conventional equipment. In such event our invention` can be applied to remove the unevenness. A suitable `tion, depending upon the choice of the opera-ter.

-tion of mud. The cutters gradually cut away the high areasof the pattern due to the percussion and chipping action of the head as the assembly is gradually lowered, until all unevenness has disappeared.

When drilling throughclay or other soft formations, the electrical actuator may or may not be put into operakluweyer keven under such conditions use of the actuator glenerally facilitates drilling, particularly in that it eliminates tendency toward balling adjacent to the drill head, and in general makes possible a higher drilling speed.

Previous reference has been made to Athe fact that the actuator is of the resonant type and is operated at a constant frequency, independent of other factors. Particlularly the frequency of impacts is independent of the rate or pressure of the circulating mud, thus making possible maintenance of efficient drilling conditions at all levels and 4independent of the mud circulating system. Furthermore with the present arrangement the pressures and tiow rates of the circulating mud can be the same as with conventional rotary drilling practice, since the equipment offers no added resistance to the circulating mud.

Although in the foregoing example we have referred to an impact frequency of the order of 1800 impacts per minute, it will be evident that this practice can be varied somewhat depending upon drilling conditions, the size of the equipment, and the like. We believe that in all instances it is desirable to keep this frequency above about 1000 impacts per minute. 1800 strokes per minute has been found satisfactory in the equipment used to date, although it is anticipated that in some applications it may be desirable to increase the frequency to say 2000 impacts per minute or more. The upper frequency limit is that which can be imparted to the drill head with relatively good cutting ei-Hciency.

We claim:

l. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head in the lower end of the housing and aligned therewith, means forming a spline connection between the head and the housing whereby the head has limited axial movement relative to the housing and is caused to rotate together with rotation of the housing, an anvil member attached to the head within the housing and extending axiallyY therewith, and an electrically energized actuator, said actuator including at least one electromagnet field assembly having a laminated core with spaced pole pieces terminating at the boundary of a cylindrical passage and having a gap therebetween and a central piston like core disposed axially of the housing radjacent the gap and reciprocated by energization of the electromagnet assembly to apply impacts to the anvil at a substantially constant frequency.

2. Drilling equipment as in claim l together with means forming passages for flow of drilling mud through the housing, about said electromagnet assembly and through ducts in the drilling head.

3. Equipment as in claim 2 together with means forming a fluid tight seal between the drilling head and the housing.

4. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head at the lower end of the housing and aligned therewith, means forming a spline connection between the head and the housing whereby the head has limited axial movement relative to the housing and is caused to rotate together with the rotation of the housing, an anvil member attached to the head within the housing and extending axially therewith, and an electrically energized actuator carried by the housing, said actuator including upper and lower electromagnetic eld assemblies each having a laminated core with spaced pole pieces terminating at theboundary of a cyclindrical passage and havingagap therebetween and a .central piston-like core disposed :axially of, the housing adjacent the gaps and reciprocated by'energization of the electromagnetic assemblies topply impacts to the anvil at asubstantially constant frequency. 5. in well drilling equipment, a tubular housing adapted to bedisposed in a well, a cutter head aligned with the housing and carried at one end of the same, an anvilmember attached to the head and extending into the housing, means serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, an actuator including at least one electromagnetic eld assemblyhaving a laminated core with spaced pole pieces terminating at the boundary of vthe cylindrical passage and having `a gap therebetween, and a central piston-like core disposed axially of the housing adjacent the gap and axially reciprocated by energization of the electromagnetic field assembly, said core serving to apply impacts to the anvil member at substantially constant frequency.

6. in well drilling equipment, a tubular housing adapted to be disposed in a well, a cutter head aligned with the housing and carried at one end of the same, an anvil member attached to the head and extending upwardly coaxially into the housing, means serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, an actuator including at least one electromagnetic field assembly having a laminated core with spaced pole pieces terminating at the boundary of the cylindrical passage and having ya gap therebetween, a central piston-like core disposed axially of the housing adjacent the gap and axially reciprocated by energization of the electromagnetic field assembly whereby the core serves to apply impacts to the anvil member at substantially constant frequency, and passages forming ducts for flow of rotary mud through the housing to the head.

7. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head aligned with the housing and carried at one end of the same, a mounting member serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, and impact transmitting bar having its lower end secured to said mounting member and extending coaxially up into said housing, guide means carried by the housing .and serving to guide said bar for axial movement within the same, a recoil spring surrounding said bar and adapted to urge the same upwardly in said housing, an electrical actuator including at least one electromagnetic field assembly and a central piston-like core disposed axially of the housing and reciprocated by energization of the electromagnetic assembly, means for guiding said piston-like core for axial reciprocating movement within said housing, said core serving to apply imypacts tothe upper end of the impact transmitting bar.

8. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head aligned with the housing andcarried at one end of the same, a mounting member serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, an impact transmitting bar having its lower end secured to said mounting member and extending coaxially up into said housing, guide means carried by the housing and serving to guide said bar for axial movement'within the same, a recoil spring surrounding said bar and adapted to urge the same upwardly in said housing, an electrical actuator including at least one electromagnetic field assembly having a laminated core with spaced pole pieces terminating at the boundary of a cylindrical passage and having a gap therebetween, and a central piston-like core disposed axially of the housing adjacent the gap and axially reciprocated by energization of the electromagnetic assembly, means for guiding said piston-like core for axial reciprocating movement within said housing, said core serving to apply ,impact to the upper end of the impact transmitting bar.

9. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head aligned with the housing and carriedat one end of the same, a mounting member serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, an impact transmitting bar having its lower end secured to said mounting member and extending coaxially up into said housing, guide means carried by the housing and serving to guide said bar for axial movement within the same, a recoil spring surrounding said bar and adapted to urge the same upwardly in said housing, au electrically energized actuator including at least one electromagnetic eld assembly and a central piston-like core disposed axially of the housing and reciprocated by energization of the electromagnetic assembly to apply impacts to the impact transmitting bar as it is reciprocated downwardly, an impact receiving member mounted within said housing above the piston-like core and adapted to receive impacts from the same as it is reciprocated upwardly, and a recoil spring serving tourge said impact receiving member downwardly.

10. In well drilling equipment, a tubular housing adapted to be disposed in a well, a rotary cutter head aligned with the housing .and carried at one end of the same, a mounting member serving to mount the head to the housing with limited freedom of movement relative thereto in an axial direction, and impact transmitting bar having its lower end secured to said mounting member and extending coaxially up into said housing, guide means carried by the housing and serving to guide said bar for axial movement within the same, a recoil spring surrounding said bar and adapted to urge the same upwardly in 10 said housing, an electrically energized actuator including at least one electromagnetic eld assembly having a laminated core with spaced pole pieces terminating at the boundary of a cylindrical passage and having a gap therebetween, and al central piston-like core disposed axially of the housing adjacent the gap and reciprocated by energiaztion of the electromagnetic assembly to apply impacts to the impact transmitting bar as it is reciprocated downwardly, an impact receiving member mounted within said housing above the piston-like core and adapted to receive impacts from the'same as it is reciprocated upwardly, and a recoil spring serving to urge said impact receiving member downwardly.

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